Tachometer running backwards

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NOLA-boy

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I have a 1993 Johnson 40hp. The tach was disconnected when I bought the boat. I hard wired the tach plug and ran it to the tach. It ran backwards for some reason. I got my multi meter and checked the wires. Purple is 12v. Ground is good. Grey is 11ish v when running. I thought the tach may be bad. I just installed a new tach with pointer on 6p and I have the same symptoms. I have driven the boat for hours since the purchase and the charging system seems to be functioning. I've always had I/O and inboards so please pardon my ignorance.
Here's a video of the tach.
Tachometer running backwards. 1993 Johnson 40hp - YouTube
Thanks in advance!
 

Silvertip

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Re: Tachometer running backwards

+12 volts and ground may be fine but are the wires on the correct pins. The gray sense wire is an ac signal, not dc so that voltage is not correct.
 

NOLA-boy

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Re: Tachometer running backwards

Oh.?gray is a/c. Thanks. What should it read when running? I imagine if its the send wire it varies with rpm? I'll change my multi meter to a/c and check it.
I'm sure the wires are correct.
Purple-Ignition
Black-Ground
Gray-Send
Thanks!!!
 

NOLA-boy

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Re: Tachometer running backwards

I changed my meter to AC.
Send wire at idle 22.2v.
Reved it a little and it reached 25.
Any thoughts??
Thanks!
 
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Re: Tachometer running backwards

I don't think the tach is running backwards. It looks more like the tach is loosing signal when you throttle up which could be a wire or connection under the flywheel that's rubbing or flexing as the timing plate moves. Check the wiring at the outboard then as a test disconnect the throttle from the carbs (this will stop the timing plate moving during the next part of the test) and being real careful not to over rev the motor just add 500 rpm by opening the butterflys on the carbs by hand (tiny amount of movement) if the tach picks up the 500 rpm difference then that shows that when the timing plate moves there is a problem with the signal wire. You can repeat the test backwards by moving the timing plate with out the carbs opening for a short period of time to see if the tach dies when the plate is moved.
 

NOLA-boy

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Re: Tachometer running backwards

Thanks so much!
I'll give it a try tomorrow morning before work if I have time.
It does seem to move up as if it would work then crash back down.
Thanks again. I'll give u a report soon.
 

NOLA-boy

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Re: Tachometer running backwards

I have performed the test and the results are the same. Same tach symptoms when just throttle or moving timing advance.
My neighbor suggested that I start the engine and pull the Batt cables and check the output.
With the cables pulled it's putting out .4v. We tried it with the Batt connected too. 12v when off 12.4 when running.
Thanks again!!
 

NOLA-boy

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Re: Tachometer running backwards

I hope I didn't mess thins up by running the motor with batt disconnected. I was just reading how that could fry the rectifier. It seems its not working properly anyway.
Thanks again for your help.
 

UncleWillie

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Re: Tachometer running backwards

I changed my meter to AC.
Send wire at idle 22.2v.
Revved it a little and it reached 25.
Any thoughts??
Thanks!

...With the cables pulled it's putting out .4v. We tried it with the Batt connected too. 12v when off 12.4 when running.
Thanks again!!

I hope I didn't mess thins up by running the motor with batt disconnected. I was just reading how that could fry the rectifier. It seems its not working properly anyway.
Thanks again for your help.

Your rectifier is dead and not sending power to the battery or pulses to the tachometer.
The 20+ volts would be normal for a disconnected battery but VERY BAD for all the electronics!!!
The battery should measure 12.5+ engine OFF, and 13.5 to 15 volts, engine running, if the alternator is working and charging the battery.
12.0 volts is a nearly dead battery. 12.6 is a full battery, Engine OFF.
 

NOLA-boy

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Re: Tachometer running backwards

Ok. Thanks so much!!
I'll replace the rectifier and have the battery load tested.
 

UncleWillie

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Re: Tachometer running backwards

Keep in mind that the problem is something associated with the Alternator/Rectifiers and not necessarily the actual rectifiers.
A bad connection causing an open ground to the alternator will give the same results.
In MAY be blown rectifiers, but then again it MAY not!
We have reasonably isolated the general area of the problem but not necessarily the the specific failure point.

I want to make sure you understand why I do not necessarily recommend blindly replacing parts just because that MIGHT be the problem.

Do you have 20+ volts AC leaving the Alternator and near 0 volts DC at the output of the Rectifier?
If there is nothing going in, nothing will come out.
 
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NOLA-boy

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Re: Tachometer running backwards

Sorry it's been so long. Busy at work. I have decided to bring the boat to the shop. I know so little about outboards that I don't know how to perform the tests that y'all have requested.
Thanks for all your help. I'll stick with working on my inboards myself and leave the outboards to the pros.
Thanks again for all of your input.!
 

Silvertip

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Re: Tachometer running backwards

Don't understand why a two stroke should be a mystery if you can work on a I/O. Spark is spark, Fuel is fuel, carburetors are carburetors and charging systems although they look different between an I/O and outboard, work essentially the same way. In fact an outboard charging system is simpler than an I/O. There is also no difference between a tach for an I/O and one for an outboard. All they do is count pulses. In the case of an outboard, it counts alternator pulses rather than ignition pulses. It is after all, an AC unit just like a car. Both are rectified (turned to DC) and then sent to a voltage regulator. The reason the outboard can count alternator pulses is because there is no belt involved so the pulses are a direct relationship to engine rpm. Belt driven alternators cannot use that process since belt slippage, pulley sizes and the variety of alternator designs would require way too many option settings on the tach. With an outboard, it doesn't make any difference if the engine has one cylinder or a dozen. All the tach needs to know is the number of alternator pulses per revolution.
 

NOLA-boy

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Re: Tachometer running backwards

I know :(
Just being lazy. I don't have a manual and I'm busy at work.
I know the timing, firing order, plug gap, fuel pressure etc? of the SBC. Mercruisers I work on by memory. Most things I do, I don't need a manual.
Maybe I'll get a manual and "man up" and learn some outboard.
Thanks for the encouragement.
I'll do some homework, get my multimeter and get back with y'all.
I know its the same concept just a different language.
Thanks again!
 
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